Each glossary topic defines a single sense of one term. Besides identifying the term and providing a definition, the topic can identify related terms. The glossary is a specialization of the concept topic.
Figure 1. DITA Glossary opened in Serna.
You can find more information about glossary topics at: http://docs.oasis-open.org/dita/v1.1/OS/archspec/glossaryArch.html.